Throughout this application various publications are referred to by Arabic numerals in brackets. Full citations for these references may be found at the end of the specification immediately preceding the claims. The disclosures of these publications are hereby incorporated by reference in their entireties into the subject application to more fully describe the art to which the subject application pertains.
Expression of the BCL6 (B-Cell Lymphoma 6) transcriptional repressor is required for B-cells to form germinal centers (GC) and undergo immunoglobulin affinity maturation [1,2]. BCL6 contributes to the GC B-cell phenotype of clonal expansion and genetic recombination by repressing target genes involved in DNA damage responses such as ATR (ataxia telangiectasia related), TP53 (tumor suppressor protein p53) and CDKN1A (cyclin dependent kinase inhibitor 1A) [3,4,5]. BCL6 can also repress the PRDM1 (PR domain containing 1, with ZNF domain) gene and thus inhibit plasma cell differentiation of GC B-cells[6,7]. Translocations or mutations of negative regulatory elements that occur as byproducts of class switch recombination or somatic hypermutation can lead to constitutive expression of BCL6[8,9]. Such events are among the most common genetic lesions found in human diffuse large B-cell lymphomas (DLBCL). Animals engineered to recapitulate deregulated expression of BCL6 in germinal center B-cells develop DLBCL similar to the human disease[10,11].
BCL6 is a member of the BTB-POZ (bric a brac, tramtrack, broad complex-pox virus zinc finger) family of proteins. Homo-dimerization of the BCL6 BTB domain forms an extended lateral groove motif along the dimer interface, which is required to recruit the SMRT (Silencing mediator of retinoic acid and thyroid hormone receptor) and N-CoR (Nuclear hormone receptor corepressor) co-repressors[12]. Amino acid side chains protruding into this groove make extensive contact with an 18-residue BCL6 binding domain (BBD) peptide that is conserved between N-CoR and SMRT[12]. The BCL6 lateral groove residues that contact N-CoR and SMRT are unique to BCL6 and are not present in other BTB proteins[12]. A recombinant peptide containing the SMRT BBD along with a cell penetrating TAT domain and other motifs was able to block interaction of BCL6 with SMRT and N-CoR. This BCL6 peptide inhibitor (BPI) could re-activate BCL6 target genes and kill BCL6-expressing DLBCL cell lines in vitro[13]. DLBCL cells thus require the continued presence and function of BCL6 for their survival, suggesting that BCL6 is a bona fide therapeutic target in this disease. Although BPI could kill DLBCL cells at low micromolar concentrations, it was highly unstable and required frequent re-administration to cell cultures in order to detect its biological activities [13]. Therefore, despite the initial effectiveness of BPI, there is a still a great need for a more potent and stable inhibitor.
Due to the importance of BCL6 in B-cell differentiation and DLBCL development, there is a need for a stable, non-immunogenic and non-toxic inhibitor specific for BCL6 capable of disrupting BCL6 repression complexes in DLBCL cells. The present invention addresses that need.